Think about the space between decking boards - too small and possum poo will get stuck, too large and keys etc will fall through! (Colleen).
The reason you have the gap is so the boards can expand and contract. The gap also depends on the type of timber because some timbers expand/contract more than others, and the width of the timber (typically wider boards expand more). If the boards are too close then when the timber expands, the timber will lift off the bearers. Having said all that, the minimum space is usually 5mm (Tony – builder).